In precision engineering, flexures are used to replace traditional mechanical joints to provide an accurate, repeatable, wear-free, and friction-free motion. Flexures have been extensively used in engineering applications and can be found in many devices such as compact-disc players, metrology instruments, and positioning devices. Flexures-based mechanisms are very attractive for applications that demand high precision because they provide high linearity with minimal friction and wear.
Compliant structures with a remote center of rotation, also referred to as “RCR” structures, have been extensively used in the automation industry as powerful tools for peg-in-a-hole types of applications. The compliance of the structures is used to self-accommodate part tolerances and alignment errors and to guide the insertion of one part into another part. Passive compliance systems are based on the assumption that there exists a geometrical reference surface that is sufficiently stiff and that can be used to drive an assembly. By definition, passive compliance mechanisms are deformable structures with preferred compliant axes.
There are many cases, however, where passive compliant systems are inefficient. This is particularly true for micro-assembly tasks that not only often lack accurate geometrical references but also require a different approach to contact-based interaction due to the scaling effect on physical interaction forces. RCRs are also inefficient for task-oriented assemblies, such as fiber alignment, in which the alignment quality metric is defined by the quality of the signal transmitted through the interface. Therefore, there is a need for a RCR positioning device that can be actuated. Despite existing RCR kinematics, no actuated RCR flexures have been developed.
In the process of optical system design, the use of optical benches with discrete components that can be quickly reconfigured is essential. In this design scope, a true gimbal mechanism (i.e., a mechanism that provides pan and tilt motions to the desired reference surface) is extremely useful. For example, such a mechanism greatly simplifies the alignment procedure for a laser beam.
One solution combines additional degrees of freedom to virtually recreate a RCR through an adequate command algorithm. For instance, with three degrees of freedom, two translations and a rotation, one can program a rotational motion outside of the mechanical structure. Therefore, with a six-degrees-of-freedom platform, a two-degrees-of-freedom rotational motion can be programmed outside of the mechanical structure. This method offers several advantages, including the ability to easily reconfigure the center of rotation. This method implies, however, the use of a controller combined with an accurate geometrical model of the structure. In addition, by definition, the system needs to be motorized and calibrated. Because the complexity of the system is high (a controller is needed) and because additional degrees of freedom are required (in space, six degrees of freedom), the cost of these systems is usually high. In addition, this system lacks compactness and requires calibration procedures.
U.S. Pat. No. 3,357,268 issued to Richter discloses an optical cell that has a center of rotation defined by a spherical shape machined on one part of the device. The device is made of several parts and uses a similar principle that is described in U.S. Pat. No. 6,198,580 and U.S. Pat. No. 4,088,396.
U.S. Pat. No. 4,088,396 issued to Edelstein discloses an optical mount enabling independent orthogonal adjustment of the angular position of an optical element. It comprises a cell adapted to receive an optical element and a base defining a chamber open on one side to receive at least a part of the cell. This device is similar to the device disclosed in U.S. Pat. No. 6,198,580.
U.S. Pat. No. 4,276,697 issued to Drake et al. discloses a compliance element for a remote center of compliance unit that is formed from a multi-stranded cable having an elastomeric collar molded around a center portion and terminating in threaded connectors for incorporation into the compliance unit. The structure is passive.
U.S. Pat. No. 4,337,579 issued to De Fazio is a deformable remote center of compliance device having a remote center of compliance (RCC) that utilizes flexures. The structure is passive.
U.S. Pat. No. 4,480,918 issued to De Fazio discloses a non-contact displacement sensing system for a RCC device having a movable part and a fixed part. The RCC uses flexures that have five degrees of freedom (bellows-type of flexures). The structure has three loops and, therefore, the mobility is four. This structure has four degrees of freedom: two rotations and two translations. The structure is passive and is parallel.
U.S. Pat. No. 4,485,562 issued to De Fazio discloses a RCC-type of structure using known kinematics. The RCC uses flexures and has a mechanism connected to it that modifies its stiffness depending on the assembly sequences.
U.S. Pat. No. 4,537,557 issued to Whitney discloses the same system described in U.S. Pat. No. 4,485,562 but uses the mechanism attached to the elastic structure to create the function of gripping. The structure is semi-active.
U.S. Pat. No. 5,419,674 issued to Chang discloses a semi-active compliance device that can be used to control the position, force, and orientation of a package during a package stacking operation. The device contains an X-Y motion mechanism for actively controlling the fine positioning and force of a package in two orthogonal directions (X, Y). The design also includes a RCC mechanism for passively controlling the orientation of the package about a third orthogonal direction.
U.S. Pat. No. 5,529,277 issued to Ostaszewski discloses a RCC-type of compliance mechanism that has two degrees of freedom. The structure is serial. Flexure pairs are positioned at right angles to each other and thus comprise perpendicular four-bar linkages.
U.S. Pat. No. 5,909,941 issued to Cheng et al. relates to a passive multiple remote center of compliance. The device is used as a wrist for a robotic arm. This design is made of several parts serially interconnected. It uses a gyrational structure for a remote center part of the structure.
U.S. Pat. No. 6,198,580 issued to Dallakian discloses an optical mount for an optical element which has an optical surface. This device is used for positioning a mirror or optics and has a center of rotation that is on the surface of the optical element. This design uses moving parts that slide into each other and are friction type of joints.
JP 11-138487 issued to Hayakawa Takahiro Natta Ind Corp. discloses an elastic structure that uses bellows arranged such that they define a RCR outside of the mechanical structure. The principle seems to be similar to the one described in U.S. Pat. No. 4,480,918.
A goniometer is based on a ball-screw mechanism. The tap is made such that the moving platform describes a circular motion relative to a remote center. Axes can be stacked to combine two degrees of freedom. This type of mechanism effectively creates a RCR. It is sensitive, however, to backlash and friction and is not easily reconfigurable. A gimbal mechanism like the one in U.S. Pat. No. 6,198,580 issued to Dallakian is based on a universal joint actuated from the side. This mechanism is also not free of friction and is very sensitive to specific dimensions such as a given thickness and size of mirror.
These technologies use bearing-type or friction-based mechanical joints. High resolution, accuracy, and repeatability can hardly be obtained with competing technologies that use bearing-type or friction-based mechanical joints. It is necessary to use flexures to obtain these performance attributes.
FIG. 1 illustrates the actuation principle of a conventional one-axis four-bar link 10. The conventional four-bar mechanism comprises four connected bars 12, 14, 16, and 18 linked to form a trapezoid. Bar 18 is the fixed part of four-bar link 10, and is fixed to the ground or reference surface 20. Bars 12, 14, and 16 pivot at each of their ends about a rotational axis orthogonal to the plane defined by any two adjacent bars. Thus, bar 14 pivots about point A, bars 12 and 14 pivot about point B, bars 12 and 16 pivot about point C, and bar 16 pivots about point D. Before any of the bars are rotated, an angle is set between bars 14, 18 and between bars 16, 18.
A line 15 may be extended from bar 14 beyond bar 12. A line 17 may be extended from bar 16 beyond bar 12. Lines 15 and 17 from bars 14 and 16, respectively, may be extended beyond bar 12 until they intersect. The point where lines 15 and 17 from bars 14 and 16 intersect defines a remote center of rotation (RCR) 1. The position of the remote center of rotation can be fully determined based on the geometrical parameters of the four-bar link 10.
In view of the discussion of conventional devices provided above, a need remains for a remote center of rotation positioning device that can be actuated. It is an object, therefore, to design such an active structure. A related object is to design a RCR structure accurate enough to define a remote center of rotation outside the mechanism in a given accuracy window. Another object is to provide a structure offering minimal wear and friction capable of repeatable motion and reconfiguration. Still another object is to use flexures and avoid bearing-type or friction-based mechanical joints. A further object is to provide a structure that can hold optical devices such as optical fibers, mirrors, and crystals. A still further object is to mount the output of the mechanism such that the free volume around the structure is maximized.